3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) and LTE-Advanced (LTE-A) standards are developed to meet growing capacity demands due to rapid expansion of wireless data services. One challenging aspect of these standards is the optimal usage of limited radio resources shared by multiple wireless devices. Specifically, the Physical Downlink Shared Channel (PDSCH) is designed to carry downlink data, and the Physical Uplink Shared Channel (PUSCH) is designed to carry uplink data, while the Physical Downlink Control Channel (PDCCH) is designed to carry Downlink Control Information (DCI) in each subframe to provide wireless devices with necessary scheduling information in terms of resource allocation, modulation and coding scheme information, and power level information for proper downlink data reception and/or uplink data transmission. The terms DCI and PDCCH are sometimes used interchangeably when there is no confusion.
In current LTE standards, there are several DCI formats including DCI formats 0 and 3 for uplink (UL) and DCI formats 1 and 2 for downlink (DL). A DCI carrying DL scheduling information is also called a DL assignment and a DCI carrying UL scheduling information is called a UL grant. The information of each DCI is rate matched and scrambled with a cell-specific and slot-specific scrambling sequence. One wireless device could have one or more DCIs in the same subframe. Each DCI is carried on one or more control channel elements (CCEs) depending on DCI length and DL radio channel condition. The number of CCEs used is often called the CCE aggregation level, which can be 1, 2, 4, or 8. An aggregation level larger than 1 means DCI payload is repeated over more than one CCE, resulting in a lower code rate, which is often needed for wireless devices in poor radio channel conditions.
PDCCH Link Adaptation (LA) intends to choose an optimal CCE aggregation level and power for each DCI for each wireless device based on the DL channel condition of the wireless device. If the channel condition is good, a small number of CCEs (a low CCE aggregation level) and/or a low transmit power may be used. Otherwise, a large number of CCEs and a high transmit power may be used. The number of control symbols available to be used for PDCCH is limited. As such, the number of available CCEs for each subframe, which are shared by all the wireless devices serviced by a network node, is also limited. That means the performance of PDCCH LA may greatly impact the LTE network performance by affecting factors such as capacity and the number of wireless devices served by a network node.
As an example, in the case of Voice over Internet Protocol (VoIP), which demands a large number of DCIs, PDCCH capacity may be a key limiting factor for VoIP capacity. If PDCCH LA is too aggressive by using a small number of CCEs for each wireless device and/or a low transmit power for each wireless device in order to support as many wireless devices as possible within each subframe, wireless devices may have more PDCCH decoding failures, meaning some wireless devices may fail to locate the related DL data sent through the physical downlink shared channel (PDSCH) or may miss UL grants for PUSCH transmission. This may result in significant throughput reduction and/or reduced user satisfaction. On the other hand, if PDCCH LA is too conservative by using a large number of CCEs or a high transmit power for each wireless device, the number of wireless devices that can be accommodated within each subframe will be smaller, which may lead to a low VoIP capacity, which is especially unacceptable in VoIP applications. As such, good PDCCH LA design is important.
The DL channel condition used in the PDCCH LA for a wireless device is based on the Channel Quality Indicator (CQI), which is determined by the wireless device and reported to the network node through UL channels such as PUSCHs or Physical Uplink Control Channels (PUCCHs). The network node will use CQI reports to estimate Signal-to-Interference-plus-Noise ratio (SINR), which, together with a target PDCCH block error rate (BLER), is used to determine PDCCH LA. This is referred to as a pure CQI report based PDCCH LA. Accurate and timely CQI reports will help the network node adjust the CCE aggregation level and transmit power. Unfortunately, accurate and timely CQI reports may be difficult to obtain due to these limitations: (a) CQI reporting cannot be too frequent, as its reporting interval is limited by signaling overhead; (b) CQI reporting accuracy may vary from one wireless device to another depending on wireless device specific implementation; (c) often, each wireless device derives its CQI by checking cell-specific reference signals, which may not necessarily take into account the interference on PDCCH regions or PDSCH resource blocks. As such, there is a strong need for an additional adjustment on the CQI reported from the wireless device. This additional adjustment is referred to as an outer loop adjustment. The outer loop adjustment done for control channel LA, e.g., for the PDCCH LA, is referred to as control channel outer loop adjustment. Similarly, there is also an outer loop adjustment done for the data channel LA, e.g., for the PDSCH LA, and that is referred to as data channel outer loop adjustment. PDCCH LA outer loop operates based on the feedback the network node receives from the wireless device about the PDCCH transmission results. A feedback indicating a failed transmission will move the outer loop output in a direction that will result in more conservative coding and/or higher power in PDCCH transmission, and a successful transmission will move the outer loop output in the opposite direction. However, in Carrier Aggregation (CA), the network node sometimes receives ambiguous feedbacks from the wireless device. That is, the network node cannot decide whether the previous PDCCH transmission was a success or a failure. As a result, the PDCCH LA outer loop is not able to function effectively when the wireless device operates in CA mode. Thus there is a need to develop new methods to tackle this problem.